Limit prompt length automatically

[jamesquilty]

I know others have raised the usability issues caused by having the full path of the current working directory as the rshell prompt. I have my terminal windows 80 columns wide, giving just enough room for two terminal windows side-by-side on my laptop. My working directories can be longer than 80 characters and the overlap this caused in rshell, along with the line munging reported in #152 (and noted in /main.py#L73-L75) was irritating me.

So, I wrote a quick loop which replaces the home directory part with ~ and then automatically trims the prompt if the directory part is still longer than [arbitrarily] 30 characters. The following change to line 1863 in the current HEAD revision on master should be made to enjoy a prompt that shows at the minimum the current directory name, even if its length is greater than 30 characters, and in all other cases displays a directory part of the prompt which is strictly 30 characters or less in length. It gives a prompt that looks like either ~/Hacks/rshell> or software/hacks/rshell>, which has been working well for me.

     def set_prompt(self):
         if self.stdin == sys.stdin:
-            prompt = PROMPT_COLOR + cur_dir + END_COLOR + '> '
+            prompt = cur_dir.replace(os.path.expanduser("~"), "~")
+            if len(prompt) > 30:
+                # Make the directory part of the prompt shorter!
+                head, prompt = os.path.split(prompt)
+                head, tail = os.path.split(head)
+                while len(os.path.join(tail, prompt)) <= 30:
+                    prompt = os.path.join(tail, prompt)
+                    head, tail = os.path.split(head)
+            prompt = PROMPT_COLOR + prompt + END_COLOR + '> '

I'd be happy to submit a pull request for this if it would be accepted.